These kinds of apps are exploding right now, appealing mainly to Millennials looking for convenient ways to start saving and increasing their money. The apps’ low barriers to entry, automation and familiar tap-swipe-buy, Tinder-style interface have led to a gold rush of sorts.

Stash lets you pick more than 40 different exchange-traded funds (ETFs) and stocks for a minimum deposit of $5. According to Stash, since it launched in late 2015, it’s amassed nearly 2 million customers and 5 million educational subscribers, with approximately 40,000 new clients joining weekly.

Acorns lets you round up credit and debit card purchases to the nearest dollar and then invests your digital change. It says it’s now up to 3 million users.

Robinhood, a stock brokerage app that lets you buy and sell individual stocks for $0 a trade, has about 3 million accounts, and more than 1 million people signed up for early access to Robinhood’s new crypto-trading service.

Acorns reminds people that ups and downs are normal. (Photo: Jennifer Jolly Special for USA Today)

The Robinhood app is free, while Acorns and Stash charge a low management fee.

One of the biggest advantages to micro-investing apps is that they let you bypass brokerage account minimums — which sometimes require thousands to start — and skip the struggle of saving up enough to invest in a mutual fund.

Download the app, set up a profile that lets the company know the best kinds of things to invest your money in, connect a bank account, and you’re off.

They’ve attracted young investors as the bull market sprints toward its 9th birthday. But they’re relatively untested in a down or very volatile market.

Several 20-something investors I spoke with say they’ve been great. What once was prohibitively expensive and especially daunting for this demographic is now literally in the palm of their hands.

An ad for Robinhood’s new Cryptocurrency trading service. (Photo: Jennifer Jolly Special for USA Today)

“Most of the apps that are coming out are essentially piggy banks,” says Chris Michaels, a Raleigh/Durham-N.C., investment enthusiast who uses a handful of similar apps on his phone. “You put a little in week after week, and before you know it, hopefully you’ve grown your bank account to something substantial.”

Many app users I spoke with say they’re adding about $25 a week into their micro-investing accounts. Not enough to really “make or break” them financially should the markets take a massive downturn. The apps give them a cheap, convenient and familiar modern way to dabble in investing by making small, automated transfers into passive, individual non-retirement accounts.

“I use them to learn about the stock market and gain perspective in general,” my stepson, Guy Blelloch, tells me over the phone.

His Robinhood account grew by about $500 in the first year — or about 15% — on his initial investment. That’s about 14.99 percentage points more than it would have earned sitting in a traditional saving account during the same time period, he points out. Of course, he could have just as easily lost it all, too.

Stash lets you pick stocks based on your core values and beliefs. (Photo: Jennifer Jolly Special for USA Today)

When the markets took a nosedive in early February, many novice investors said they were worried. On Facebook group pages, micro-investing app users encouraged each other to “ride the wave” and invest for the long term. Acorns and Stash also sent in-app messages reiterating the importance of riding out the rough patches.

A standard refrain from experienced financial advisors: Don’t put anything on the table you’re not willing to lose.

“These companies have an opportunity and an obligation to really educate users,” says Robert Barba, senior banking and fin-tech analyst at personal-finance website Bankrate. “You’re investing in the market, and the market swings. These are not usually sophisticated investors. Plunges can lead to panic, overreaction and risky behavior that you don’t usually see with more (seasoned investors).”

The text message from my stepson that started it all – an add for Robinhood. (Photo: Jennifer Jolly Special for USA Today)

Do-it-yourself investors need to do their homework, making sure they take advantage of an employer-matched 401(k), figuring out how seemingly small fees cut into returns and seeing if they have access to tax-favored investment accounts like an IRA.

“If you’re in a financial position to use these apps, that is, if you’re already saving 15% or so of your income for retirement, you’ve already built up an emergency fund, and you’ve paid down high-interest debt, then they can be a great way to learn about the markets,” Andrea Coombes, NerdWallet’s investing and retirement specialist says.

“But the money you put in should be your play money. You should be willing to lose it if that (a market crash) happens.”

The final warning comes from both people using micro-investing apps and the financial experts watching from afar: Be careful not to get too swept up.

Several users told me that instead of obsessively checking their social media feeds — they’re now obsessively checking their stocks. For smartphone users who already have a hard time ignoring Snapchat, Facebook and text message notifications, this is one more way to get hooked.

How they work

Robinhood offered a free stock on sign-up. It doesn’t charge fees to maintain an account, transfer funds or buy stock, though there’s a regulatory fee for sell orders.

I downloaded the app on my iPhone and followed the prompts to fill-in a bunch of information: Home address, Social Security number and link to a bank account. That was actually my first hurdle. Putting so much sensitive information into an app on my smartphone creeps me out.

Once I finished getting it all set up, which takes less than five minutes total, I let the app pick a stock for me at random. As of right this moment, I own one share of SIRI (Sirius XM) worth $6.32. Okay, I think, now what?

Stash lets you pick stocks based on your core values and beliefs. (Photo: Jennifer Jolly Special for USA Today)

I repeated this process with Acorns and Stash. Acorns is easy to “set and forget,” free for college students for four years with a valid .edu email address. Or you pay $1 a month until your account balance hits $5,000, then 0.25% of your account balance per year.

Stash turns out to be my personal favorite because it’s loaded with educational information and lets me find stocks that align with my core values with options like “Clean & Green,” “Equality Works,” and “Live Long & Prosper.”

The first month of account management is free. After that, you pay $1 a month in management fees for accounts under $5,000 and 0.25% on higher balances.

While I’ll sit for at least a year on the investments I’ve now made into Robinhood, Acorns and Stash, they’re not the kind of thing I’ll be using for much more than saving some spare change at the end of the day — kind of like the more modern-day version of my old spare change jar.

Then again, I’m not the target demographic. My stepson and his friends are. For them, the key seems to be using these apps more as learning tools than get-rich-quick schemes.

Jennifer Jolly is an Emmy Award-winning consumer tech contributor and host of USA TODAY’s digital video show TECH NOW. Email her at jj@techish.com. Follow her on Twitter @JenniferJolly.